To characterize Dexter in the correct manner, I think you need to look at him like a poisoned flower. By that, I mean that he looks completely normal from the outside, and you wouldn’t think that he would hurt anybody.
Just like a poisoned flower, he looks normal, maybe even looks good but on the inside, he is as cold as ice.
Dexter is a very smart guy, both in his methods of killing and his ability to reenact the human emotions, even though he doesn’t have any, according to himself.
One of the obvious signs that he is extremely intelligent, is his job. Usually, when you see a guy working for the police, who helps solve murders, you wouldn’t think of him as an evil person. You would probably think the opposite.